1. Field of the Invention
The present invention relates to a developing device utilized for an electrophotographic copying machine, a facsimile, a printer, or the like, and more particularly, to a developing device using a two-component developer having toner particles and carrier particles.
2. Description of Related Arts
In the field of image forming apparatuses that form images by electrophotographic systems, developing devices using a two-component developer having toner particles and carrier particles have been known. Such a developing device is arranged in close proximity to a photosensitive drum or the like in order to develop an electrostatic latent image formed on a surface of the photosensitive drum into a toner image. The developer in a suitable amount is supplied to the surface of the photosensitive drum by a developing roller provided in the developing device, and the toner in the developer is selectively electrostatically attracted by the electrostatic latent image formed on the surface of the photosensitive drum when the developer is rubbed onto the surface of the photosensitive drum. Accordingly, the electrostatic latent image on the photosensitive drum is developed into the toner image.
Meanwhile, in the conventional developing device, the density of the developer supplied to the photosensitive drum is not uniformly distributed, as viewed in the axial direction of the developing roller. Accordingly, the density of the developer at both ends in the axial direction of the developing roller is liable to be in a higher concentration than that at the center thereof. Specifically, the thickness of the developer magnetically adsorbed on an outer peripheral surface of the developing roller is larger at both the ends in the axial direction of the developing roller than that at the center thereof (the height of the developer from an outer surface of the developing roller is large).
Therefore, the surface at both ends in the axial direction of the photosensitive drum is more strongly rubbed by the developer, as compared with the other portion, whereby the photosensitive layer at both the ends of the photosensitive drum is roughened. When a photosensitive layer is roughened, the concentration at both ends of an image to be formed is reduced, or a background of the image is fogged, for example.
An object of the present invention is to solve such a problem and to provide a developing device that prevents a photosensitive layer from being easily roughened in a part of its surface, particularly at both its ends.
Another object of the present invention is to provide a developing device improved such that the thickness of a developer is not larger at both ends in the axial direction of a developing roller than that at the center thereof.
The present invention is directed to a developing device for developing an electrostatic latent image into a toner image using a two-component developer having toner particles and carrier particles, characterized in that the length of a draw-up magnetic pole is made shorter than the length of a main magnetic pole such that both ends in the axial direction of the draw-up magnetic pole are respectively inward in the axial direction by a predetermined length from both ends in the axial direction of the main magnetic pole.
The present invention is directed to a developing device for developing an electrostatic latent image into a toner image using a two-component developer having toner particles and carrier particles, characterized in that the space between a control plate for restricting the thickness of the developer to a predetermined thickness and an outer peripheral surface of a sleeve is made narrower in regions at both ends in the axial direction of the control plate than that in the other region.
Generally, the developer magnetically adsorbed onto the outer peripheral surface of the sleeve by the draw-up magnetic pole extends toward both ends of the sleeve beyond the length of the draw-up magnetic pole when it passes through the control plate. When the sleeve is rotated to a position over the main magnetic pole in the state, a magnetic force at both ends of the main magnetic pole attracts the developer extending toward both the ends of the sleeve. Therefore, the density of the developer over at both the ends of the main magnetic pole, that is, the thickness of the developer adsorbed on both ends of the outer peripheral surface of the sleeve is increased.
In order to solve such a problem, in the present invention, the length of the draw-up magnetic pole is made shorter than the length of the main magnetic pole. Therefore, the amount of the developer adhering to both ends of the outer peripheral surface of the sleeve is reduced, so that the density of the developer at both the ends of the sleeve is maintained in suitable conditions in a state where the developer is opposite to the main magnetic pole.
The length of the draw-up magnetic pole is thus made shorter than that of the main magnetic pole, thereby narrowing a range in which the developer is put, as viewed in the axial direction, on the outer peripheral surface of the sleeve before passing through the control plate, to prevent the developer from excessively extending toward both ends in the axial direction of the sleeve when the developer passes through the control plate. Consequently, both ends of a photosensitive layer are prevented from being excessively rubbed, thereby making it possible to prevent both ends of the photosensitive layer from being roughened or worn. Accordingly, the image quality at both ends of a formed image is not degraded. As a result, the life of a photosensitive drum can be improved.
In the present invention, out of the developers magnetically adsorbed on the outer peripheral surface of the sleeve by the draw-up magnetic pole, the thickness of the developer adhering to both the ends in the axial direction of the sleeve is restricted to decrease. That is, the thickness of the developer in a portion other than both the ends in the axial direction of the sleeve is restricted to a predetermined thickness, and the thickness of the developer at both the ends of the sleeve is restricted to a thickness smaller than the predetermined thickness. Therefore, the developer which has extended toward both the ends of the sleeve is attracted by both the ends of the main magnetic pole when it is over the main magnetic pole. However, the thickness of the developer at both the ends of the main magnetic pole is restricted to a small thickness. Accordingly, the developer is attracted by the main magnetic pole, so that the thickness of the developer is approximately equal to the thickness at the center of the main magnetic pole.
Accordingly, only the thickness of the developer at both the ends of the sleeve can be prevented from being increased.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.